Commercially available dicarboxylic acids are obtained through a limited number of processes which are well known to those skilled in the art. Hence, carboxylic acids are generally produced through oxidation of saturated or unsaturated hydrocarbon substrates such as fatty acid substrates using either a gaseous component such as air and ozone, a nitric acid/metal catalyst component or a combination of both components. Numerous prior art documents disclose variants of such oxidation processes.
In the case of carboxylic acid production using air, British Patent 809,452 assigned to Celanese Corporation of America describes a process for the preparation mono and diacarboxylic acids. The process comprises the air conversion of a mixture of hydroxy fatty acids having up to 10% by weight concentration in its unsaturated counterpart. Also, U.S. Pat. No. 2,813,113 granted to Emery Industries describes the production of azelaic acid from unsaturated fatty acids having 10 to 24 carbon atoms. The process consists in a two-step ozone-air oxidation of straight chain unsaturated fatty acids such as oleic acid. Ozone as an oxidant is highly reactive, selective and gives rise to relatively clean reaction products with minimal residues or by-products. Nevertheless, ozone production requires high technology equipment, large capital investments with great annuity costs. Its concentration (1.5-2% ozone by weight) implies a large volume of gases to be processed that negatively influences the oxidation kinetics, requires high costs and strict safety control.
A number of prior art documents describe variants of air-oxidation processes. For example, U.S. Pat. No. 4,328,365 discloses a process for the oxidation of hydrocarbons using a carbon monoxide-reduced vanadium pentoxide catalyst immobilized on an inert support. U.S. Pat. No. 2,136,144 describes a process for the conversion of cycloalkenes to acids using oxygen and a vanadium catalyst. Also, U.S. Pat. No. 3,388,157 describes a process for the air oxidation of alkanes. This process, which appears to involve the use of high temperatures, requires elaborated separation systems using distillation and oxidation.
Oxidation of fats and fatty acids with nitric acid and oxides of nitrogen is one of the oldest reactions in chemistry. Nitric acid is an inexpensive strong oxidant. It has however only been used commercially to a limited extent because of the non-specificity of the resulting reaction products and the difficulty in controlling the reaction.
Oxidation of unsaturated fatty acids with nitric acid is more specific than the oxidation of saturated fatty acids but it still generates a mixture of dibasic acids. For example, oxidation of oleic acid with 85-95% HNO.sub.3 at 20.degree.-25.degree. C. for 3-8 hours gives 44-56% azelaic acid, 16-17% suberic acid, 7-12% pimelic acid, 6-10% succinic and glutaric acid, 3-4% adipic acid and 0-2% sebacic acid. Oxidation of conjugated linoleic acid and tall oil fatty acids gives an even wider distribution of dibasic acids and total lower yields than with pure substrates and the reaction is extremely slow.
Still, many prior art references describe methods for producing dicarboxylic acids which are essentially one-step methods for oxidizing aliphatic or aromatic substrates using a strong acid such as nitric acid in the presence or absence of a vanadium catalyst. Hence, in U.S. Pat. No. 2,323,861, a process is described for converting unsaturated hydrocarbons such as cyclohexenes to dibasic acids by reacting the cyclohehexene substrate with nitric acid and a variety of catalysts including ammonium vanadate. Also, one step reaction of 88% nitric acid and 0.1% ammonium metavanadate catalyst on unsaturated substrates has been reported in U.S. Pat. No. 2,203,680. However, the reaction conditions were such that it took two days at room temperature to obtain 50% diacid products.
In U.S. Pat. No. 2,343,534 assigned to du Pont, there is described a process for the industrial production of adipic acid. In this process, a cyclohexane/cyclohexanol combination is reacted in the presence of nitric acid and an ammonium metavanadate/copper nitrate catalyst mixture. Also, azelaic acid and other diacids and monoacids have been prepared by direct action of chromic acid and sulphuric acid on oleic acid as described in U.S. Pat. No. 2,450,858. Also, in German Patent 3,016,225, there is described an oxidative post-treatment for purifying di or polybasic carboxylic acids. The acids are initially produced by reacting unsaturated hydrocarbons with nitric acid.
The combined use of air and acid/catalyst components has also been reported in the literature. In U.S. Pat. No. 1,991,188, phthalic acid is produced by treating either o-xylene or naphthalene with nitric acid in the presence of oxygen gas and vanadium pentoxide. U.S. Pat. No. 2,662,908 refers to the treatment of 24 carbon atom fatty acids with air in the presence of dilute nitric acid (8-30%) at high temperature.
Oxidation processes involving both air and nitric acid are also described in U.S. Pat. No. 2,791,598. The document discloses a process by which saturated aliphatic hydrocarbons are converted to organic acids by oxidizing the saturated aliphatic hydrocarbon substrates with air or other oxygen-containing gases and nitric acid. A catalyst such as vanadium pentoxide can also be used. In U.S. Pat. No. 2,978,473, a process is described to prepare dibasic acids from hydrocarbon substrates. In this process, the hydrocarbon substrate is oxidized with air in the presence of a metal catalyst and submitted to water extraction and separation prior to reaction with nitric acid. This process appears to be complex and requires many purification steps.
In one-step oxidation reactions using either a nitric acid/catalyst component, a gaseous component or a combination of both, the major drawbacks are random attack on the substrate and rearrangement reactions which give rise to low yields of a mixture of dibasic acids. Furthermore, impurities substantially lower the yield of the desired final products. Also, direct action of nitric acid on unsaturated substrates leads to the formation of nitroso and nitrate compounds. These reactions compete with the oxidation reaction, which lowers the yield of the diacid products as well as producing a multi-component mixture.
In summary, current oxidation processes for the production of diabasic acids from hydrocarbon substrates suffer from low yields because the reaction usually lacks specificity and does not discriminate from one possible oxidation site to another. There appears to be a need for improved processes allowing a more efficient targetting of the desired oxidation site in the substrate.